Tag Archives: Archaeological Geophysics

One of the more constraining factors of geophysical survey in an archaeological context is the potential difficulty in dating or phasing anomalies in the data. Although hard science forms the main component of survey work, there is a large subjective element involved in the interpretation of geophysical survey data, which ultimately can decide the nature, function and phase of features. Our interpretation sometimes belies the complexity of the archaeological remains in question (issues relating to survey resolution and the nature of deposits can affect this) and this is particularly true of the different phases of construction and occupation at a site. Complementary information on the phases of deposits represented in geophysical survey data can be provided through surface collection and recording over the survey area, and this integrated approach to archaeological survey is particularly pertinent to the recent Portuslimen survey conducted at Ephesus.

In August and September of 2015 a small team of archaeologists from the University of Southampton travelled to Ephesus in Turkey, to take part in the season of fieldwork run by the Österreichisches Archäologisches Institut (ÖAI). The ÖAI has conducted archaeological research at Ephesus for 120 years (http://www.oeai.at/index.php/grabungsgeschichte.html ), and for the last 16 years has conducted geophysical survey across the site, with work directed by Dr Sirri Seren of the Central Institute for Meteorology and Geodynamics (https://www.zamg.ac.at/cms/en/news). The Portuslimen team were invited to look at some very specific areas of the ancient site and landscape, namely the areas of the inner and outer Roman harbours, and the surrounding landscape of the river. The aim of the survey was to add to the areas already surveyed by the ÖAI, and to address the development and possible phasing of the harbour complexes of ancient Ephesus. The team comprised the author, together with Dominic Barker, Ben Urmston, Jack Pink and Jack Frowde.

Panoramic view of the area of the outer harbour and river as seen from the promontory

Magnetometry being undertaken at the inner harbour

To address these aims the team utilised fluxgate gradiometers, with gridding out and topographic survey conducted using an RTK GPS. Magnetometry was used in a field to the east of the inner harbour of Ephesus, and in fields around the outer harbour and river channel, also on the promontory overlooking the outer harbour.

In addition Electrical Resistivity Tomography (ERT) profiles were conducted at sites along the northern edge of the river channel, across the area of land between the outer harbour and river, and in the area of the floodplain to the north of the inner harbour. These were designed to assess the nature of the deeper sediments associated with the geoarchaeology of the landscape, for comparison with the borehole data collected by the University of Koln.

Although the geophysical survey techniques were deemed appropriate for the conditions of the site, the broad geographical range of the different survey areas,and the potential variations in phasing for the different results, presented the team with a problem; how to recognise the different periods of occupation in these areas, together with an idea of the character and function of many of the structures. To address these issues, fieldwalking was undertaken over each of the survey areas., utilising the grid set out for the geophysics, and using each of the survey markers as the central point for an area of surface material to be collected and recorded within a a 3m radius.

Set out of the survey grid using an RTK GPS

ERT survey to the north of the river channel

Surface collection to the north of the river channel, using the RTK GPS and note-taking

The method of sampling allowed collection of all pottery, with a record of presence and absence of CBM (? meaning )and building material. The collection of surface ceramics provides the diagnostic sherds necessary for establishing the type and chronology of the vessels, while its total collection over a small but standard sample area make it possible to calculate the count and weight of sherds and, thus, the potential ceramic density density in each area.

The results of the survey not only provided some excellent information about structures and port infrastructure in the different areas, but also provided some useful spot-dating of the potential phases of occupation of the areas. On the promontory overlooking the outer harbour a number of pit and gully features were found in the geophysical survey results, matching other surface evidence for Archaic settlement pre-dating the Roman harbour. The results of survey to the north and west of the outer harbour indicated large structures associated with the port, dating to the Roman and Late Antique periods. Finally the survey results to the north of the modern river channel detected structures and a possible canal that, when compared to the surface collection, seem to represent Late Roman and Byzantine phases of the port of Ephesus.

Location of surface collection points across the 2015 survey area

Roman amphora bases located to the west of the outer harbour

The results of the season are only preliminary, and further work needs to be undertaken on the data and at the site to fully understand the complexities of the changing harbours of Ephesus. However, a pattern is emerging which seems to indicate a gradual shift in the development of port infrastructure, with activity moving from the inner and outer harbours between the early Roman and Late Antique periods, to a later complex to the north of the modern river between the Late Antique and Byzantine periods. The evidence for pre-Roman occupation in the area is also of interest.

There are obvious caveats about the methodology that we have applied to the site, such as the depth of potential deposits and the nature of material exposed at the surface, which will ultimately affect the dating of material to particular periods of occupation. However, the combined strategy of surface collection and geophysical survey does provide us with a more nuanced data-set than with geophysics on their own, and has already started to raise interesting questions relating to the development and use of this fascinating port complex.

Over the past few months of field survey, work has swung around to a series of projects lined to Roman ports in the Mediterranean. In June and July I headed back for a season of excavations at Portus, and in August and September geophysical survey at the site of Ephesus in Turkey (more on this in a future post). In October and November work has commenced on a geophysical and topographic survey of the Roman port area of the town of Tarragona in Catalonia, Spain. The work, together with that of Ephesus, forms part of the Roman Meditteranean Ports (RoMP) project, or Portuslimen (http://portuslimen.eu/). A component of this project involves the survey of a number of port sites, to understand the form and extent of these sites, and help analyse the ways in which they may have functioned through time. The work at Tarragona is being conducted in collaboration with colleagues from l’Institut Català d’Arqueologia Clàssica (ICAC).

The modern port of Tarragona, with its harbour, docks and factories

The issue with Tarragona is that the development of the town and port spans from the pre-Roman period, to the establishment of the Roman town in the third century BC, to Late Roman and Visigothic settlement in the 6th, 7th and 8th centuries, and later Islamic influence. Over this time the town and its port expanded and contracted, and archaeological deposits were buried under fluvial deposits from the Francolí river to the west of the port. Later post-medieval expansion of the town, particularly in the 19th and 20th centuries saw much of the Roman and later port built over, and a new harbour constructed over the remains of the ancient port. The aim of our survey is to attempt to locate significant structural remins of the Roman port and harbour through intrusive methods. The good news is that a number of areas in the city have been excavated in the past 40 years, which helps in the location of the survey to gain as much as possible from the efforts. The bad news is that, due to the modern town plan, many of the survey areas are constrained by modern buildings and infrastructure. Thus our work has to use some specific techniques to conduct survey in the areas that are available. This includes topographic survey and location of survey grids using RTK GPS and total station survey, and use of GPR and ERT along streets and in plazas to map buried remains.

Set up of the GPS and GPR by the University of Southampton team in the Placa dels Carros

The GPS base station collecting static data for the survey

Fortunately for the University of Southampton team, the open plazas of the town provided sufficient space for static data to be collected by the GPS base station, and for a series of preliminary stations to be established using the GPS. Where the streets became narrow a total station was used to establish further stations in a traverse around the port area of the town.

Geophysical survey s far has focused on 500MHz GPR, propagating 3-4m below the modern street level to find the buried archaeology. In some of the streets in the northern part of the port area, particularly along the roads close to the Roman baths and theatre a number of walls and other features are visible. As the survey progresses southwards, however, modern infrasructure such as manhole covers, and the nature of the made-up ground close to the modern harbour, make the results more difficult to interpret. The restrictions in terms of spatial coverage have also provided a challenge in terms of data interpretation.

Total station survey in one of the streets of Tarragona

500MHz GPR survey in one of the plazas

To better understand the geoarchaeology of the site we have been applying Electrical Resistivity Tomography (ERT) to record both archaeology and deeper deposits in the town. The main constraint with this is that the paved areas of the town preclude the use of survey probes (these cannot be dug through cement and Tarmac). Thus we have had to adapt using a system of electrode copper conductors and a conductive gel. Most surfaces will allow an electrical current to be passed through them, but asphalt and Tarmac act as insulators, meaning that for the ERT to work long stretches of cement pavement need to be surveyed.

A conductor formed from crocodile clips, wire and pipe end copper usually used for plumbing.

The ERT profile being conducted adjacent to the Roman theatre

In spite of our initial misgivings, the conductors and gel, with the ERT equipment, have proved to work very well indeed. The profile alongside the Roman theatre has revealed a number of areas of walls and rubble aligned adjacent to the excavated theatre remains. The team hope to conduct a long profile of ERT in the southern part of the modern port which, together with the boreholes of Ferreol Salomon, will investigate the nature of the harbour deposits.

The survey work is being conducted until 15th November, and there are many features of the topography of the ancient harbour that remain to be discovered, including the line of the Roman seafront and mole.

After the successes of the surveys and excavation at Basing House in 2014, a second season of work is being conducted by the Basing House CAT project (http://basinghouseproject.org/) directed by Nicole and Gareth Beale. Work on the excavation is ongoing, and can be seen on the project blog. In addition to this work, however, further geophysical survey is also being conducted on Basing Common.

Elliot surveying the possible location of the siege camp on Basing Common using a magnetometer, with Basing House within the trees in the background

A combination of geophysics and metal detecting is being used over the area to provide information on the location of the Parliamentarian siege camp established in the area during the siege of Basing House. Work started today with Dominic Barker, the author, and a team of students and volunteers. Dom and others involved in the survey will be posting blogs in the coming weeks. However, the survey started well with a grid being established in the southern part of the Common.

Dom Barker gridding out using a GPS

A small area of magnetometry was covered, however, the results seem to indicate the presence of possible anomalies relating to a possible camp, including a broad ditch feature, a possible bastion, and other more ephemeral ditches and pits. The ploughsoil also indicates ferrous material over the area possibly associated with artefacts from the seige. The plan is to use metal detecting to find artefacts across the survey area, with these being bagged up and located using the GPS, allowing their distribution to be compared with the geophysical survey results. Please check back for further developments over the duration of the field season.

Due to commitments in the field over the last month or so it has proved difficult to keep up to date with the blog. Now seemed like a good time to produce a few posts to highlight some recent fieldwork and site visits, starting with a recent survey at Naukratis. In May 2014 I conducted geophysical survey at this archaeological site in the Nile Delta. The Naukratis fieldwork project is directed by Ross Thomas of the British Museum, and seeks to assess the surviving archaeology of this important ancient site using a range of complementary methods including topographic and geophysical survey, in addition to borehole survey and excavation, as part of a larger project directed by Alexandra Villing of the Greek and Roman department of the British Museum called ‘Naukratis: Greeks in Egypt’.

Magnetometer survey being conducted to the east of Kom Ge’if

The season in May (the third season of fieldwork) added to the existing dataset from the first two seasons in 2013, mapping the extent of the ancient settlement and its association with the Canopic Branch of the River Nile.

The port of Naukratis was the earliest Greek port in Egypt, established in the late 7th century BC as a base for Greek (and Cypriot) traders and the port of the royal Pharaonic city of Sais. It was an important hub for trade and cross-cultural exchange long before the foundation of Alexandria and continued to be significant through the subsequent Ptolemaic, Roman and Byzantine periods. Previous fieldwork was conducted by Flinders Petrie amongst others, and concentrated on excavation of the central areas of the ancient town. Further research was required to fully understand this very important archaeological site. For this season, the magnetometer survey of the site was continued in the fields surrounding the modern village.

Two ERT profiles were also undertaken using an Allied Associates Tigre ERT. The first of the profiles ran from a point some 400m to the west of the site, over the kom or mound, to a point 400m to the east of the site, incorporating the line of the Canopic Branch of the Nile and Naukratis. This provided a section 15m deep running west-east across the southern part of the site. The main aim was to better understand the geological relationship between the river and the settlement, and to tie the profile in with the series of borehole surveys conducted along the same traverse.

Mohamed Roshti assisting with the ERT survey on the kom

The survey work at Naukratis has produced significant new data on the layout of the ancient town, its local environment and hinterland, including the location of the Greek sanctuary complex, the Hellenion, and the Temenos or temple enclosure at the site. The magnetometer results located a large number of mud brick and stone structures in the fields around Kom Ge’if, particularly in the north and east of the site. In addition to the plan of the ancient town, the magnetometer results also give us a much better idea of the extent of the ancient site in relation to the location and development of the Canopic branch of the Nile, which ran to the west of the ancient settlement. The magnetometry clearly shows the change from settlement to canal infilling, with structures positioned along the edge of the canal. This data is reinforced by the results of the ERT survey. The depth of deposits underlying and surrounding Kom Ge’if is suggested by the topography, with a sharp contrast between the kom and the surrounding fields. The remains of the ancient site are present, if buried, but life in the modern village of Kom Ge’if carries on. The village, with its new mosque, stands out from the surrounding floodplain, a mixture of bean fields, and brown ploughed and saturated fields prepared for planting melon. A number of venerable sheikhs’ tombs stand out on the fringes of the village. The local shepherd crosses from field to field, allowing his sheep and goats to graze on the stubble remaining from the wheat harvest, and manuring the fields in the process, then herds the flock back through the winding streets of Kom Ge’if.

Detail of survey results from Naukratis

Many of the areas of the ancient settlement still require surveying using magnetometry, and a combined strategy of ERT survey with drilling of boreholes will provide useful comparative data for particular parts of the site and its hinterland.

David Knight undertaking magnetometry in the vicinity of the Tiber levee on the Isola Sacra in 2008

The new discovery of extensive urban remains to the north of the river Tiber at Ostia Antica http://www.portusproject.org/blog/2014/04/new-city-wall-discovered-ostia/#.U063XyX5rTc.twitter highlights part of the survey project conducted between 2008 and 2012 across the Isola Sacra, the area of delta between Ostia Antica and Portus. The Isola Sacra, defined by the modern line of the River Tiber to the south and east, the Fossa Traiana to the north, and the Tyrrhenian Sea to the west, covers some 250 hectares, although the prograding nature of the Tiber Delta means that the ancient Roman coastline lies some 1.5km inland from its modern counterpart, reducing the area to 150 hectares.

The Isola has been a relatively unknown area relative to the sites of Portus and Ostia Antica. Although excavation of the Necropolis di Porto in the 1940s revealed the line of the Via Flavia connecting Ostia Antica with Portus, and tombs dating from the 2nd, 3rd and 4th centuries, the ‘sacred island’ has been the focus of scant investigation, beyond archaeological findspots and more recent investigation for mitigation works (see gazetteer in Germoni et al. 2011). Survey work between 2008 and 2012 focused on the production of a high resolution magnetometer survey of the landscape, covering most of the area between the ancient coastline, close to the line of the Via della Scafa, and the Tiber and Fossa Traiana. This survey was not a hunt for possible extensions of urban areas across the Tiber or the Fossa, but an attempt to understand the formation of this part of the delta in terms of geoarchaeological material, and the settlement and use of the zone, particularly for the Republican and Imperial periods, and the development of both Ostia Antica and Portus.

Results of the magnetometer survey in the southern part of the Isola Sacra. Satellite image courtesy of DigitalGlobe

Seven seasons of survey work were conducted over the area, representing a massive effort by staff and students at the University of Southampton, the University of Cambridge and the British School at Rome, directed by Prof. Simon Keay and Prof. Martin Millett. Gridding and topographic survey was conducted with total station and RTK GPS, allowing the entire landscape to be gridded prior to the geophysical survey being conducted. Bartington Instrument fluxgate gradiometers were then used to survey across the landscape, covering an area of 150 hectares, to add to the 260 hectares covered at Portus in previous seasons, picking up on faint variations in the Earth’s magnetic field caused by buried archaeological deposits.

RTK GPS being used to survey in the vicinity of Ostia Antica by the University of Southampton and BSR team

The survey deepened our understanding of the complexity of archaeological deposits across the Isola, indicating the presence of significant waterways and canals associated with Portus and the Isola Sacra, together with a system of smaller canals for drainage and irrigation, or pre-Roman or Roman date. Compared with other data such as satellite imagery, a full assessment of the nature of archaeological features was conducted (Keay, Millett and Strutt 2014). In addition targeted use of geophysical survey methods has elucidated on the presence of archaeological remains, including the presence of Late Antique walls and structures close to the Episcopio near Portus, found during GPR survey in the area.

MSc students from the University of Southampton carrying out a GPR survey in the vicinity of the Episcopio, between Portus and the Isola Sacra

The results of the geophysics shows traces of the formation of the delta in the area of the Isola, including magnetic features associated with the prograding nature of the delta, and previous flow of the Tiber. This is particularly marked on the Isoal close to the Fiume Morto or ancient bend in the Tiber close to Ostia Antica. The significant canal that runs from Portus appears to run all of the way across the Isola, arriving close to the modern east – west course of the Tiber.

The canal visible in the south of the Isola Sacra (from Keay, Millett and Strutt 2014)

It is in this area that the substantial remains of an extension to the town of Ostia was found. The results indicate the presence of warehouses and large municipal buildings, some measuring 140m by 80m in length and breadth. The structures are enclosed by the line of a large defensive wall, with rectangular towers, each measuring 8m across.

Detail from the magnetometer survey results from the southern part of the Isola Sacra

The findings of the urban area are exciting in their own right. However what is more edifying is the relationship between this area and the surrounding hinterland of this complex port system. Beyond the gates of the city lies a landscape of fluvial and canal features linking the wetland of the delta, with its salt pans, fields and marble marshalling areas, with the River Tiber and the surrounding settlements and farms of the Tiber Delta. The results of the survey present an image of a complex and nuanced port and fluvial system, requiring considerable further analysis to understand the full significance of the delta area in relation to its principal urban areas and the surrounding landscape.

Keay, S., Millett, M. and Strutt, K. 2014, The Canal System and Tiber Delta at Portus. Assessing the Nature of Man-Made Waterways and their Relationship with the Natural Environment . In Journal of Water History, 6, 1, 11-30.

Over the last few weeks a team from the University of Southampton has been working with a team from the British Museum, surveying the archaeological site at Ras Al Hadd. The focus of the survey work was to carry out a topographic survey of the site, and to conduct magnetometer and GPR survey of areas of the occupation mound, prior to the commencement of excavation of the site. In addition the geophysical survey grid was used as the basis for intensive surface collection across the western portion of the site.

The fort at Ras Al Hadd

The Bronze Age and Iron Age archaeology at the site forms a mound on the coastal plain a few kilometres from the tip of the south-east limits of the Arabian Peninsula. Later settlement has covered most of the site, including a substantial fort and new mosque. The remnants of post-medieval defence overlie the mounds of the later prehistoric settlement, one of a number in the area of the coast. It is the later archaeology that is immediately striking upon visiting Ras Al Hadd. A number of old cannon litter the village, some propped on breeze block plinths, some lying forgotten. Others are located in the fort, and lines of Martini-Henry rifles line the walls of the entrance to the courtyard. In the vicinity of the inlet and lagoon to the west of the site, a solitary tower guards the entrance to the inlet and the reaches of the lagoon.

The lagoon to the west of Ras Al Hadd

The focus of the survey was, however, the prehistoric material at Hadd. The survey utilised a number of techniques to map the area. The survey grid was established using RTK GPS, with the instrument also being used to map the topography across the site and the surrounding coastal plain between the mound and lagoon. Surface features were also surveyed in for comparison with the geophysical survey results.

GPS survey under way

Phil Riris carrying our magnetometer survey

The site was also surveyed using magnetometry and GPR, focusing on the western and northern parts of the mound,where modern occupation has affected the archaeology the least, and within the interior of the fort. Results of the survey revealed a number of potential archaeological features, including remains of earlier phases of the fort, Iron Age pits, and cairns. In addition to the geophysics, the British Museum team and a team of workmen undertook a systematic surface collection of a part of the surveyed area. The preliminary results show the strength of the methodology incorporating such techniques, with the plot of the fieldwalking overlying on the geophysics, and giving some clear pointers as to the nature of the ancient deposits.

Fieldwalking being carried out along the western side of the mound

One of the outcomes of travelling in the area was the opportunity to take a cursory look at the coastal area with a radius of a few kilometres from Hadd. The terrain shows the same plain, covered with thorn scrub and coastal inlets, but with a number of small occupation mounds breaking the topography. Indicating quite a dispersed settlement and use of different sites by the population, from prehistory up until more recent times. An amazing landscapewith hopefully much further scope for work.

Further to my last post, the week before last was spent with a team from the British School at Rome carrying out a single Electrical Resistivity Tomography (ERT) profile across part of the Isola Sacra, designed to complement the magnetometer survey of the area, and recent coring undertaken by Ferreol Salomon.

The Necropolis di Porto, showing the Via Flavia, between Portus and Ostia Antica. The ERT survey was carried out in an effort to cover the area of the necropolis, ancient road and dune cordon along the Roman coastline, together with the features including a large canal to the east of the necropolis

The profile ran from the westernmost point of the Necropolis di Porto in a north-east direction along one of the principal trackways of the Isola Sacra, along the edge of the fields established by the bonificazione of the area. Levels of readings were taken down to a depth of 7m to match the sediments recorded in Ferreol’s cores. The short field season highlights a useful way of targetting features found in the broader blanket survey of the landscape to aid our understanding of the depth and nature of features located in area datasets. Alice James and Matt Berry from the British School at Rome assisted in the survey on the ground, and are now hard at work processing this data and ERT data from some other projects conducted in the Autumn.

The profile along the trackway to the east of the necropolis. 180m were covered in total.

A preliminary look at the survey data shows that the tombs of the necropolis are indicated as high resistivity anomalies, although the road surface is not clear – perhaps indicating the robbing out of paving – and a high resistivity surface of material is visible running across the landscape to the east.

Data collection using the Tigre and laptop. Some battery life issues curtailed the work on the first day, although the heads-up imaging of the data as it is collected is useful in the field.

The data further to the east, in the area of the canal, is more difficult to assess on the basis of a cursory glance. The high resistivity readings near the surface have a double break in them, correlating approximately with the canal width. However, the sandy infilling sediment, and the sandier subsoil in the area, coupled with waterlogging across the survey area, has made visualisation of the canal profile difficult. Further processing and clipping of the data may give a clearer indication of the canal profile, and its relationship to the nectropolis and Via Flavia to the west.